In a power semiconductor device utilized in an inverter or a converter for an elevator, an electric railway, or the like, switching loss has a large influence on an increase in consumption energy and in the size of a heat radiation fin. That is why reduction of switching loss in a power semiconductor device is required.
Switching loss in a power semiconductor device is expressed by the product of the current and the voltage across the drain and the source thereof (in the case of a Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET); however, in the case of an Insulated-Gate Bipolar Transistor (IGBT), switching loss in a power semiconductor device is expressed by the current and the voltage across the collector and the emitter thereof; hereinafter, unless noted in particular, description will be made by taking a MOSFET as an example) at a time when the power semiconductor device turns on or turns off. It is preferable that in order to reduce switching loss, the power semiconductor device is switched at high speed so that the product of the current and the voltage across the drain and the source thereof becomes small; however, in a turn-off mode, a large change in the current (hereinafter, referred to as “dI/dt”) across the drain and the source of the power semiconductor device causes an parasitic inductance in the circuit to produce a large surge voltage, thereby breaking the power semiconductor device. A large change in the voltage (hereinafter, referred to as “dV/dt”) across the drain and the source and dI/dt, produced by switching of the power semiconductor device, cause large radiation noise; therefore dI/dt and dv/dt are limited and high-speed switching is not readily be performed.
The switching speed undergoes a large influence of the input capacitance of a power semiconductor device. The input capacitance is given as the summation of the gate-to-source parasitic capacitance and the gate-to-drain parasitic capacitance of a self-arc-extinguishing power semiconductor device. By charging and discharging the input capacitance, the switching of the self-arc-extinguishing power semiconductor device is performed. The simplest method for adjusting the switching speed is to adjust the gate resistance; however, this method causes both dV/dt and dI/dt of the power semiconductor device to change. Accordingly, due to the restriction of noise and a surge voltage, the gate resistance and the switching speed should be made large and low, respectively; thus, the switching loss increases.
In a conventional driving circuit, by adding a capacitor between the gate and the source of a power semiconductor device and adjusting the gate resistance so that the switching speed becomes equal to that at a time when no capacitor is inserted, dI/dt and dV/dt are controlled individually so that a low-loss turn-on mode is realized (e.g., refer to Patent Document 1).